In engineering applications, the data between users enters into the network via access devices, in order to implement data interaction. FIG. 1 is a schematic diagram of data interaction by user through a point-to-point access network. As shown in FIG. 1, under normal conditions, user 1 and user 2 establish communication with each other through service devices and links; said links mainly comprise access point link 1, service network, and access point link 2. In case the links are faulty, i.e., either the access point links or the service network are faulty, the communication between user 1 and user 2 will be interrupted. For important users, in case of such faults, the spare network has to be initiated to ensure normal transmission of important data. The spare network may be in any form, e.g., an additionally leased link, or a wireless communication or satellite communication link established between user 1 and user 2. In case the access point link is faulty, the user can detect link fault and initiate the spare network, trying to communicate with the remote user through the spare network. However, at that time, the remote user does not know the link fault and still communicates through the original network; as a result, the communication between the two users will be in a disconnecting state. In case the service network is faulty, neither of the users knows the network is out of service; as a result, the communication will be disconnected. If the link state can be delivered to the remote user, i.e., link state passing-through function can be achieved, the remote user can initiate the spare network and reestablish the linking with user 1 or user 2 through the spare network.
There are diverse ways to inform the remote user of link fault. A typical method is: user 1 and user 2 inform the linking information at the local to remote peer through periodical peer-to-peer inform; in case there is no linking information sent from the remote user for long, it indicates the link is faulty; in that case, the user stops the peer-to-peer inform of linking information and automatically initiates the spare network to communicate. Another typical method is: user 1 and user 2 detect the using of the spare network in real time; as long as the using of the spare network is detected, the other party will also switch to the spare network.
Though above methods can solve the problem to a certain degree, they have some drawbacks: first, normally the periodical peer-to-peer inform of linking information will occupy network bandwidth and result extra network overhead; second, in case of network congestion, the peer-to-peer inform of linking information will be delayed and may result in misoperation due to a wrong judgment; third, detection of the spare network will occupy some bandwidth of the spare network and result in extra overhead of the network; fourth, when the links recover to normal, it is a trouble that how the two users leave the spare network and return to the working network.